The invention is connected to laboratory technology and relates to a device for measuring optical density, such as a photometer, nephelometer or a fluorometer, for measuring the optical density of several samples placed on one and the same sample plate. The device can be used for instance in clinical laboratories, where analyses are made in large series. The most widely used plate is a so-called microtitration plate, which contains a matrix of 8.times.12 sample wells in a 9 mm division.
In order to render the sample plate measurement time as short as possible, multichannel devices are produced. For microtitration plates, there are available photometers provided with 8 or 12 channels for measuring one vertical or horizontal strip at a time (for example the analyzers MULTISKAN and EMS Reader MF by Labsystems Oy). Devices with as many as 96 channels have been produced. The total measurement time achieved by means of multichannel photometers is about 5 s. In practice, a sufficiently short measurement time can be considered to be 10 s.
In multichannel photometers, there is needed a number of detectors, optical devices connected to the detectors, as well as preamplifiers, said number corresponding to the number of channels. In order to distribute the light from the light source to the separate channels, there are needed optical devices. Generally it also is necessary to prevent light from flowing between channels during measurements. The processing of parallel measurement results also requires special arrangements. Thus a multichannel photometer necessarily becomes relatively complicated and expensive. In addition, the separate measurement channels are never quite equal, which may lead to harmful errors in accurate measurements.
There also exist single-channel readers for microtitration plates (for example the BIOSCREEN and Auto-EIA analyzers by Labsystems Oy), which are simpler and cheaper in comparison. However, their drawback is a longer measurement time (about 30 s). Although the measurement time need not be an essential factor from the point of view of the overall analysis, users regard the extra delay time as a problem.
The intensity of the light source fluctuates, and this is taken into account in the measurements. A typical stability requirement is of the order 1:1000. In some photometers, the light source intensity is measured only once, prior to the measurement of the samples. Before measurement, these devices must be allowed to be stabilized sufficiently long with the light source switched on. A typical stabilizing time is about 10-30 minutes, which slows down the use of the apparatus. Therefore, some devices are provided with a separate reference channel for observing the fluctuations of the light intensity and for taking it into account when calculating the measurement results. When employing such devices, the measurement can be carried out immediately after the device is switched on. A drawback, however, is caused by the additional arrangements required by the reference channel. The realization of a sufficiently accurate and stabile reference channel also calls for special arrangements.
Changes in the background signal are also taken into account in the measurements. They are caused by the bottom signal of the electronic components (including the detector) which changes as a function of temperature. Background measurement, as close to real-time as possible, is important particularly when measuring high optical densities, and the quality of the background measurement has a direct effect on the scale of the measurement area. A typical requirement for accuracy is of the order 1:100000. Many photometers employ a light interrupter, whereby the light obtained from the light source is interrupted, for instance at a frequency of about 300 Hz. Thus the point of interruption can be used for measuring the background.